In the particle beam therapy, therapy is implemented by irradiating a charged particle beam onto a diseased site, which is a therapy subject, so as to cause damage to diseased tissue; the particle beam therapy is one of the broad-sense radiation therapies. Among radiations, a charged particle beam such as a proton beam or a heavy ion beam is different from a conventional γ-ray or an X-ray in that the dose delivery thereof steeply becomes maximum in a position that is deep in a body. The peak of a dose delivery is referred to as a Bragg peak; the position (attainable depth) where it occurs is determined by the energy of a charged particle. That is because a charged particle gradually loses its energy as it passes through a substance and it delivers many doses when its kinetic energy becomes zero. Accordingly, in the particle beam therapy, by controlling energy distribution, not only the planar shape but also the depth-direction irradiation coverage can be controlled. Therefore, in order to deliver a sufficient dose to a diseased tissue while suppressing exposure of the peripheral tissues, there is required a particle beam therapy system that can appropriately control planar and depth-direction irradiation coverage (referred to as an irradiation field, hereinafter).
Among beam irradiation methods for a particle beam therapy system, in an irradiation method which is utilized often now and is referred to as a broad method, a thin beam supplied from an accelerator is enlarged and then is made to penetrate a collimator for forming a planar shape and a bolus for forming an energy distribution so that an irradiation field is formed (for example, refer to Patent Documents 1 through 4). In this situation, in the bolus, when a beam penetrates the bolus, the energy is attenuated in accordance with the penetration length (thickness); therefore, a thickness distribution is set in such a way as to compensate the depth distribution from the body surface of an irradiation subject. For example, when the distal plane of an irradiation subject is the setting reference, regarding a beam as a parallel light, the thickness of the bolus is set in such a way that the distal plane enters the bolus, or strictly speaking, in such a way that a constant thickness is obtained when the bolus is superimposed on the tissue ranging from the distal plane to the body surface. Alternatively, taking the spread from a point light source into consideration, the thickness of the bolus is set in such a way that a constant thickness is obtained, when the bolus which has been enlarged at a predetermined magnification in the planar direction is superimposed on the tissue.